This invention relates to an improved method for plasma sterilization, the practice of exposing articles to be sterilized to a gas discharge plasma and in so doing sterilizing medical and dental instruments for re-use.
Modern medical and dental practice involves the use and re-use of certain instruments which cannot withstand the high temperatures and steam pressures historically used to sterilize instruments. Such autoclave sterilization worked, and continues to work, for stainless steel and metal instruments. Newer instruments requiring re-use which cannot be sterilized using an autoclave without damage are fiber-optic devices, e.g., angioscopes, bronchioscopes, endoscopes, and proctoscopes--ductile plastic devices, metal products which corrode, rubber gloves, gowns, sutures, syringes, and catheters.
One sterilization technique used for such temperature sensitive devices involves exposure of the articles to pressurized ethylene oxide (C.sub.2 H.sub.4 O) in a leak-proof, shatter-proof container. The ethylene oxide must permeate the entire article for effective sterilization. Articles must be exposed to these conditions for up to three hours to achieve the desired level of sterilization. Ethylene oxide is toxic and carcinogenic to humans, so this permeation in turn requires an aeration period within the container of at least twelve hours. This process is considered time-inefficient, and, more importantly, because of the toxins employed and the air exchange required for complete aeration, dangerous.
Another such technique utilizes gamma radiation or electron beams with 5 to 10 Mev of energy. This technique, like the others, works as a batch process in that articles to be sterilized are put in a chamber of some type for some period of time. Both gamma-ray and electron beam devices are expensive due to the high cost of gamma sources and electron accelerators and the complexity of the safety system required for shielding of the operator from radiation. Certain important medical instruments are adversely effected by repeated exposures. For example, latex rubber cures and loses flexibility, and catheters become brittle.
Plasma sterilization addresses many of the above concerns. Low heats are used, often less than 150 deg. F. The gaseous plasma is not toxic or carcinogenic. Sterilization can also be accomplished in a reasonably short time. Radioactive sources are not necessary and therefore monitoring and disposal of sources is not required. Expensive safety and shielding is not necessary.
Plasma sterilizers are well known commercial apparatuses. New plasma sterilization chamber designs have been the subject of patents (e.g., U.S. Pat. No. 5,393,490). These apparatuses and the latest designs suffer from certain commercially important deficiencies, however. Most such sterilizers do not generate a homogeneous plasma density throughout the chamber, resulting in relatively long batch exposure times to ensure acceptable sterilization. The plasma density is also low, further lengthening the times. These low densities and poor homogeneities often dictate the use of special sterilizing gases, peroxides, for example, in order to shorten the sterilization time. The use of such gases adds expense. Recent patented designs use radio frequency (RF) or very high frequency (VHF) discharges requiring generators of electromagnetic radiation, entailing expense and requiring protection of servicing personnel from electromagnetic radiation.
Plasma sterilizer designs can be divided into two groups:
1) Those in which the plasma is generated in a separate chamber or in a small part of the sterilizer volume and is spread by diffusion into the chamber containing the articles to be sterilized (e.g., U.S. Pat. Nos. 3,948,601, 5,115,166, 5,413,760, 4,818,488, 4,898,715, 4,931,261, 5,451,368). This design results in losses of both charged and chemically active neutral particles of the plasma to the elements of the device and reduction of the intensity of the ultraviolet radiation. This design also results in significant non-homogeneity of plasma density due to losses of charged particles onto chamber walls and onto the instruments to be sterilized. Some designs employ a screen to allow only electrically neutral plasma particles into the sterilizing chamber (e.g., U.S. Pat. No. 5,413,760) since, in the opinion of the authors, the sterilization is accomplished by only the electrically neutral plasma particles. Such screening results in substantial lengthening of the sterilization time; PA1 2) Those in which the plasma is generated directly within the sterilization chamber (e.g., U.S. Pat. No. 4,643,876, 4,818,488, 5,200,146). The devices described use RF (radio frequency) or VHF (very high frequency) fields for the generation of the plasma. H-type discharge in a skin layer near quartz or pyrex chamber walls is described in U.S. Pat. No. 4,643,876. E-type discharge actuated between an electrode inside the chamber and the chamber walls is described in U.S. Pat. No. 4,818,488. Microwave resonators are described in U.S. Pat. No. 5,200,146. The disadvantages of these devices is that the plasma density and homogeneity are disrupted by the type and amount of articles in the sterilizer, making the time to full sterilization uncertain. There is also added cost and complexity with use of RF and VHF generators and the related circuitry. Further still, in the case of the H-discharge and microwave fields, additional heating of articles can occur.
Sterilizers employing direct current (DC) glow discharge at low pressure are described in Jap. Pat. Nos. 53-35715, 60-58662. In all cases the discharge is direct: the electrons emitted from the cathode are accelerated directly towards the anode. The articles to be sterilized are simply placed between the cathode and anode and, thus, in the path of the electrons. Electrons emitted from the cathode do not spend all their energy in ionization and excitation of the gas. In addition, the articles disturb the natural electron path and, therefore, the discharge glowing, causing longitudinal and radial non-homogeneity of the plasma. These systems exhibit low energetic efficiency due to low utilization of the energy of the electrons emitted from the cathode. As with the RF and VHF designs described above, the plasma density and uniformity of these DC-glow designs are sensitive to the amount and type of articles to be sterilized, leading to longer batch times to ensure complete sterilization.